Glycols typified by propylene glycol are used as a polyester raw material, an organic solvent, a reaction intermediate or the like, and are an important substance on a chemical industry.
Propylene glycol has been synthesized by hydrating propylene oxide so far.
Propylene being a raw material of propylene oxide is a fossil-resources-derived compound prepared by using naphtha as a main component. From a viewpoint of tackling recent environmental problems such as global warming, a development of a method for producing propylene glycol without using the raw material derived from the fossil resources has been desired.
In such a situation, a method for producing glycol from hydroxyketone without using propylene as a raw material (see Patent literature No. 1), or a method for producing glycol from polyhydric alcohol through hydroxyketone (see Patent literature No. 2) have recently been proposed. In the methods, a multi-step reaction is applied and a yield is insufficient. Thus, the method has left room for improvement in view of efficiency.
On the one hand, methods for directly producing propylene glycol from glycerol under hydrogen coexistence have already been reported (see Non-patent literature Nos. 1 to 4, Patent literature Nos. 3 to 5). In these methods, propylene glycol is considered to be directly formed by using a copper catalyst having both catalytic ability of a dehydration reaction for converting glycerol into hydroxyacetone and catalytic ability for hydrogenating the formed hydroxyacetone, or the like to allow a two-step reaction to progress in one reaction system. However, selectivity and a yield of propylene glycol obtained are not sufficient. In particular, conversion is low in a liquid-phase reaction, and the methods have been inefficient (see Non-patent literature No. 2). Moreover, reaction pressure is high, and the methods have had a problem of pressure resistance required for a reaction apparatus. On the other hand, a method for carrying out a reaction under atmospheric pressure has been also disclosed, but under a current situation, selectivity and a yield of propylene glycol are not sufficient in a similar manner (see Non-patent literature No. 3), or a large excess of hydrogen is required (see Non-patent literature No. 4).